A substrate, like the one described summarily above, constitutes a fundamental component of the structure of an ink jet printhead, and in particular a printhead, also called thermal type, which is operatively based on the principle of activating the emission of droplets by heating the ink contained in the printhead itself.
This substrate has on its surface a plurality of ejection actuators which, whenever the substrate is built into the structure of the relative ink jet printhead, are arranged each adjacent to a corresponding nozzle of the printhead, and are intended to be struck by the ink contained inside the latter-named.
During use of the printhead, these ejection actuators, when they are excited impulsively with an electric current, behave as dot-like heat sources as they generally take the form of micro-resistors.
In this way, the ejection actuators heat rapidly and transfer the heat thus generated to the ink striking the substrate and the actuators themselves causing, in the immediate vicinity of each of these the formation of an ink vapour bubble which, by expanding, results in the emission of an ink droplet through the corresponding nozzle.
In general these substrates are made by way of a complex manufacturing process, starting from a silicon wafer, in which they have built into their structure the ejection actuators or micro-resistors which, as stated above, cause the generation of vapour bubbles, and thus the emission of ink droplets.
These substrates are also made in such a way as to integrate and produce certain parts of the hydraulic circuit whose role is to convey the ink to the micro-resistors, and typically they comprise a slot, made through the thickness of the substrate, which has the function of putting the micro-resistors zone into communication with a tank of the printhead containing a primary store of ink.
These substrates also integrate in their structure the electric tracks and the terminals that connect the ejection actuators with the drive circuits or “drivers” whose role is to drive them, namely to feed them impulsively with the electric current for causing ejection of the ink droplets.
Furthermore, in the most modern and sophisticated embodiments, the substrates can be produced in such a way as to integrate more complex circuitry parts, such as for instance the very drive circuits mentioned above, or significant portions of the latter.
In detail, these drive circuits are made of elements such as transistors, memories, etc., while their function is to selectively drive, in response to given external signals corresponding to a text to be printed, the micro-resistors constituting the ejection actuators, so that they heat up and generate bubbles.
Accordingly, the nozzles corresponding to the micro-resistors activated selectively emit in succession a plurality of ink droplets, so as to obtain, by composition of the print dots formed by the drops of ink, the desired printed text.
It is obvious therefore how the manufacture of these substrates is of great importance in the context of the entire printhead manufacturing cycle.
First and foremost, it is essential that the substrates are produced with great care and precision, and in strict compliance with the technical specifications, in order to guarantee that the printheads on which the substrates are mounted are capable of providing the desired performances and results, both in terms of their service life, and in terms of the printing quality obtainable with the printheads.
Likewise it is essential to produce substrates that are affected by the lowest possible defects rate, both during and at the end of the relative manufacturing cycle, to avoid their being rejected with an obviously negative impact on their production cost.
Various possible arrangements are known for these substrates, in relation both to the type of the printheads, i.e. black or color, on which they are mounted, and to the total number of nozzles included in the heads.
In the case of a color printhead, the nozzles intended for emitting ink droplets of a certain color are usually grouped into one or more banks of nozzles, distinct from the other banks of nozzles corresponding to the other colors, in which each bank of nozzles is easily discernable on the outer surface of the printhead.
In this case too, the ejection actuators are arranged and grouped on the surface of the substrate in corresponding banks, called actuating banks for clarity's sake, adjacent to a respective slot, in such a way that the ejection actuators of each actuating bank can receive the ink of the corresponding color.
Clearly the greater the number of nozzles used, the greater the number of corresponding ejection actuators, and therefore the more complex the structure and configuration of the substrate integrating the ejection actuators and relative drive circuits.
Just to complete the picture, these substrates are manufactured using the typical integrated circuit manufacturing technologies, and are generally made in multiple form starting from a round plate or wafer of silicon, the surface of which is subdivided into a plurality of cells, each corresponding to one substrate.
More particularly, various layers of different materials are deposited on the surface of this plate in successive-steps, before being selectively etched to produce the drive circuits and the ejection actuators.
The slots are then made through the thickness of each of the cells of the substrate, by way of a mechanical operation, such as sand-blasting.
Finally these plates are sub-divided into the elementary cells to form many units, each corresponding to a substrate.
As already said, it is extremely important that these substrates are made compliant with given tolerances, possibly even very stringent ones, so that no defects are revealed during the production cycles, for if this were the case, as already explained, they would be rejected automatically, obviously increasing the production costs.
Unfortunately, it often happens that the configuration adopted for the known substrates has characteristics which, at least potentially, could render the substrate manufacturing cycle particularly critical, and in particular foster the occurrence of defects during the cycle.
For example, a configuration comprising a plurality of slots aligned exactly with one another, in the lengthwise direction, tends to render the substrate particularly fragile local to the zones of separation between the slots.
In actual fact, with this configuration it is easy—potentially at least—for the substrate to break or for cracks to form along the zones of separation in a direction parallel to that of the slots.
Furthermore, by adopting a configuration of this type, with the slots aligned longitudinally, it becomes extremely important to limit as much as possible the extent of the zones of separation between the slots, in order to reduce the overall dimensions lengthwise, and thus the cost, of the substrate, and indeed also to limit the total cost of the printhead.
On the other hand, excessively reducing the zones of separation between the slots renders especially critical and delicate, and therefore easily subject to defects, the operation of hermetical sealing, local to these zones, between the hydraulic circuits intended for conveying the different color inks, i.e. that sealing operation the object of which is to avoid the various hydraulic circuits of the printhead, each corresponding to a given slot and to a given color ink, coming into communication with one another.
In fact, as will be easily understood, it is extremely critical during the assembly of the printhead to perform the operation of hermetical sealing in the zones of separation between the slots, where these zones are of limited extent.
Thus a configuration of a substrate in which the slots are aligned longitudinally is clearly not capable of optimally satisfying the conflicting requirements outlined above.
Also known from U.S. Pat. No. 5,030,971 is a substrate having a plurality of slots for the passage of ink, which extend parallel to one another in a given direction, a plurality of actuators arranged along the edges of the slots, a plurality of drive circuits, connected to the actuators, which occupy portions of the surface of the substrate extending parallel and adjacent to the slots, and a plurality of terminals, for driving the drive circuits, which are arranged in a zone above and beneath the slots and are also aligned in a direction perpendicular to the given direction of extension of the slots and drive circuits.
This configuration, though it avoids having the slots arranged in a row lengthwise, is not entirely free of drawbacks, and for instance has the terminals in not necessarily the most convenient or optimal configuration, for the purpose for instance of assembling the various parts comprising the printhead that the substrate is built into.
In particular, the terminals of this substrate, aligned in a direction perpendicular to the orientation of the slots and arranged in a zone above and beneath the rows of nozzles, could imply a somewhat tortuous and complicated configuration of the tracks of the flexible cable connecting the terminals to the corresponding contacts, arranged on the surface of the printhead, the function in turn of which is to electrically connect the latter with the outside, and in particular with the control circuit of the printer that the printhead is removably fitted in.